In current wireless LAN service, network access authentication is performed in such way that an MAC address list of a wireless LAN Access Point (AP) is registered in an authentication server managing all service subscribers instead of the wireless LAN access point, and the authentication service authenticates the MAC address transferred when a wireless terminal device accesses the network.
Meanwhile, since a wireless LAN service can be used by a smart phone and the like having an mobility as well as a notebook, it is required to provide a roaming service which can maintain an access when a user moves to another wireless LAN access point after accessing a wireless LAN access point. Further, in order to prevent an illegal use through a theft of user identification information, a simultaneous access for the same user identification information should not be allowed.
Accordingly, in order to provide the roaming service effectively, re-authentication procedure in the case of roaming should be performed considering prevention of the simultaneous access above described, and there should be no session interruption for seamless roaming service.